1. Field of the Invention
The present invention relates to signal processing and, more particularly, to a method for determining the echo distance in an acoustic pulse-echo ranging system.
2. Description of the Related Art
In level measurement applications, pulse-echo ranging systems, such as time-of-flight ranging systems, are commonly used for determining the distance to a target object (e.g., reflective surface of a material in a container) by measuring how long it takes to receive reflected pulses or echoes after transmission of a burst of energy pulses. Such devices typically use ultrasonic pulses or pulsed radar or microwave signals.
Generally, acoustic pulse-echo ranging systems include a transmitter for transmitting the energy pulses and a receiver for receiving the reflected energy pulses or echoes. Here, the transmitter and receiver may be combined in a single unit. An echo profile is generated from the received energy pulses by amplifying, filtering and envelope shaping, where at some point an analog-to-digital conversion is performed. Echo pulses are identified in the digital echo profile by a signal processor, and the distance or range of the target is calculated based on the transmit times of the transmitted energy pulses and the identified echo pulses. The calculated distance or range is then transmitted over a communication network to a process control system using a communications protocol, such as analog 4-20 mA, HART, PROFIBUS or FOUNDATION Fieldbus.
U.S. Pat. No. 7,334,470 to Bartoli et al. discloses an acoustic pulse-echo ranging system that has a fully digital receiver. Here, the raw echo data are digitized directly, i.e., without prior analog filtering, at high speed and then digitally processed to generate distance measurement. The digital processing comprises buffering, low pass filtering, bandpass filtering, envelope detecting, resampling, logarithmic amplifying, scaling and echo processing.
U.S. Pat. No. 6,856,576 to Preston discloses an echo processing method using autocorrelation. Here, the raw echo data are sampled, digitized and high-pass filtered. A correlation signal is created by adding the filtered signal to a copy of the filtered signal shifted by a time unit. A set of correlation signals is created by repeating the process for a range of time units corresponding to a set of sequential sample points. The correlation signal having the highest correlation strength is identified, and the time shift used to create it is identified as the time of flight of the echo pulse.
Raw echo data that are sampled and digitized at a high sampling frequency without prior analog filtering contains a maximum level of information, such as phase information and/or presence of multiple frequencies. This information, however, will be lost if the digitized raw data are filtered or smoothed during their further digital processing.
On the other hand, correlation of a high-resolution acoustic echo pulse comprising an amplitude modulated carrier with a similar wave form results in another amplitude modulated signal which is just as complex to evaluate. Thus, direct correlation will not help any further.